In certain electrostatographic imaging and recording processes such as electrophotographic copying processes, an electrostatic latent image formed on a photoconductive surface is developed with a thermoplastic toner powder which is thereafter fused to a substrate. The fusion step commonly involves directly contacting the substrate, such as a sheet of paper on which toner powder is distributed in an imagewise pattern, with a fusing member such as a heated roll. In most instances as the powder image is tackified by heat, part of the image carried by the sheet sticks to the surface of the roll so that as the next sheet is advanced, the tackified image partially removed from the first sheet partly transfers to the next sheet and at the same time part of the tackified image from the next sheet adheres to the heated roll. Any toner remaining adhered to the heated surface can cause a false offset image to appear on the next sheet that contacts the fusing roll and can also degrade the fusing performance of the fusing surface.
To prevent toner offset, many expedients have been tried such as providing the fusing roll with an adhesive surface such as a thin layer of an elastomer, e.g., a silicone polymer of low surface energy. Also, polymeric release liquids, e.g., polydiorganosiloxane release oils such as polydimethylsiloxane release oils have been applied to the fusing roll surface, i.e., the roll cover. With such materials, however, problems can occur.
A major problem is the effect that the polydiorganosiloxane release oils can have on the fusing roll. Although such oils aid in preventing toner build-up on the roll, they cause another problem because they are compatible with silicone polymers such as polydimethylsiloxane elastomers that are widely used as fusing roll covers. The polydimethylsiloxane oils are absorbed by the polydimethylsiloxane elastomer layers in the rolls upon repeated use and cause swelling of the rolls.
Because of the swelling of the rolls, defects appear in thermally fixed images. In particular, "step patterns" appear in the images when varying copy sheet sizes are used. These result from the differential swelling of the fusing roll inside and outside of the paper path, which causes non-uniform roll compression when different sizes of copy paper are used. There can also be increased wear on the roll and shortened useful fusing roll life because of softening of the roll surface and degrading interaction of polydimethylsiloxane oil with the core of the roll or with adhesive interlayers. The swelling problem has been controlled in fusing rolls by using fluoroelastomers that are resistant to absorption of the polydiorganosiloxane release oil. For example, U.S. Pat. No. 4,430,406, issued Feb. 7, 1984, discloses that the swell problem is controlled by overcoating a polydiorganosiloxane elastomer layer with a layer comprising a fluoroelastomer such as a crosslinked fluorosilicone elastomer or a fluoropolymer based elastomer, e.g., a vinylidene fluoride-based elastomer containing hexafluoropropylene as comonomer. The patent points out that such a fluoroelastomer layer is substantially more resistant to polydiorganosiloxane release oil absorption and, therefore, substantially decreases the forming of "steps" in an underlying polydiorganosiloxane elastomer layer. Examples of the use of crosslinked fluoroelastomers in outer layers or fusing roll covers that are not subject to the aforementioned swell problem and are adhesive to electrostatographic toners include U.S. Pat. No. 4,199,626, issued Apr. 22, 1980; U.S. Pat. No. 4,264,181, issued Apr. 28, 1981 and U.S. Pat. No. 4,272,179, issued Jun. 9. 1981.
Although crosslinked fluoroelastomers are effective to control the swelling problem in electrostatographic fusing members, such fluoroelastomers often exhibit lower than desirable thermal conductivity, which leads to inefficient heating of the toner to be fused. The addition of thermally conducting fillers such as aluminum oxide to prior art crosslinked fluoroelastomers to improve the thermal conductivity of fuser members containing them is often ineffective. Thus, the concentration of fillers needed for adequate thermal conductivity can detrimentally affect other properties of the fuser roll. For example, the rolls lose compliancy and increase in hardness which can result in inadequate fusing of the toner and undesirable toner offset. Also, if the fusing member is internally heated such as an internally heated roll, insufficient thermal conductivity can require the use of high temperatures that can cause thermal degradation, especially at the interface of the fuser roll core and an adjacent fluoroelastomer or polydiorganosiloxane elastomer layer.
It would be desirable to fabricate a fusing member that provides the benefits of a fusing member comprising a crosslinked fluoroelastomer and also exhibits a combination of properties, particularly thermal conductivity and thermal stability that is superior to prior art fusing members comprising crosslinked fluoroelastomers. An objective of this invention is to provide such a fusing member and, in addition, a fusing device employing such a fusing member.